Comp shingles on 2:12 to less than 4:12 pitch.

On any product that has thick butts like shakes or tile, shortening the exposure can actually lesson the surface pitch enough that it does more harm than good.

I don't see how any underlayment that doesn't self seal, like felt, gives you much advantage to have two layers with lots of holes vs one layer with lots of holes.

egg Said: If we eliminate wood and we eliminate angle laps and french locks (which lasted unbelievably long) and confine the topic to 3-tab strip shingles, then before any morphing could have taken place they would all have been English-measure, normally layed five inches to the weather. Getting double coverage out of them...assuming a minimal headlap...would mean laying them at less than two and a half inches to the weather. It would also mean youd go through four distinct layers of comp anywhere you cut a hole except in the area of the headlap where you would go through five.

They used to make a mineral surface roll roofing that was half-lapped. It was before my time, but not much before, and I never came across one in reroofing. In that case, as it was described to me, the bottom half was granulated and the top was smooth. I would take a wild guess and say that half-lapped morphed into double coverage and didnt refer to shingles.

Wood shingles are typically sixteen inches long and when installed normally yield a three-ply wood roof with a one inch headlap. The term double coverage therefore would not really pertain to them either, unless you layed them at less than two and a half inches to the weather. I have also seen many wood roofs run with eighty per-cent of normal exposure on low slopes. But when you start bunching up material as they do in those specialty pseudo-thatch roofs you have to start using some serious nails abd on anything low-slope that would not be a good idea.

When the manufacturers, et al, say double layer, they also usually specify layed shingle-style meaning you lay thirty-inch stock eighteen to the weather, not two layers stacked and two subsequent layers stacked on the two-inch line as most homeowners and handymen seem to choose to do when they read and attempt to comply with the instructions.

When it comes to underlayment I agree with those that say water ought not to be getting there in the first place, but I put it on anyway, and I have seen the benefits over time of using it. There was a year when one major manufacturer provided letters stating that its use was entirely optional, although they rescinded that in pretty short order. I might even have that letter in my files, but producing it would be a challenge Im not up for right now.

Opinions are like radiant energy, they radiate in all directions...I got hammered during that year for not using your fire felt. Golly gee. The only thing that really sweats is the poor guy trying to make sense of it all. One of the things I liked so much about Bill Fricklas was that when he addressed a subject at hand, he would take it all on directly, all the truths, half-truths, misapplied truths, and just plain untruths.

If we eliminate wood and we eliminate angle laps and french locks (which lasted unbelievably long) and confine the topic to 3-tab "strip" shingles, then before any morphing could have taken place they would all have been English-measure, normally layed five inches to the weather. Getting double coverage out of them...assuming a minimal headlap...would mean laying them at less than two and a half inches to the weather. It would also mean you'd go through four distinct layers of comp anywhere you cut a hole except in the area of the headlap where you would go through five.

They used to make a mineral surface roll roofing that was "half-lapped." It was before my time, but not much before, and I never came across one in reroofing. In that case, as it was described to me, the bottom half was granulated and the top was smooth. I would take a wild guess and say that half-lapped morphed into double coverage and didn't refer to shingles.

Wood shingles are typically sixteen inches long and when installed normally yield a three-ply wood roof with a one inch headlap. The term "double coverage" therefore would not really pertain to them either, unless you layed them at less than two and a half inches to the weather. I have also seen many wood roofs run with eighty per-cent of normal exposure on low slopes. But when you start bunching up material as they do in those specialty pseudo-thatch roofs you have to start using some serious nails abd on anything low-slope that would not be a good idea.

When the manufacturers, et al, say double layer, they also usually specify "layed shingle-style" meaning you lay thirty-inch stock eighteen to the weather, not two layers stacked and two subsequent layers stacked on the two-inch line as most homeowners and handymen seem to choose to do when they read and attempt to comply with the instructions.

When it comes to underlayment I agree with those that say water ought not to be getting there in the first place, but I put it on anyway, and I have seen the benefits over time of using it. There was a year when one major manufacturer provided letters stating that its use was entirely optional, although they rescinded that in pretty short order. I might even have that letter in my files, but producing it would be a challenge I'm not up for right now.

Opinions are like radiant energy, they radiate in all directions...I got hammered during that year for "not using your fire felt." Golly gee. The only thing that really sweats is the poor guy trying to make sense of it all. One of the things I liked so much about Dick Fricklas was that when he addressed a subject at hand, he would take it all on directly, all the truths, half-truths, misapplied truths, and just plain untruths.

I also tore off lots of low slope roofs that had wood shingles that had 3-1/2" to 4" exposure. Came across a few old 3-tab that were done on 4" exposure. I know for at least the past 30 years all the manufacturers called for double lapped 15 lb. on a deck under 3:12. I think tinner666 used to install laminates on some of his roofs at 4" exposure plus lap the ends if I recall.

Used to be "double coverage" referred to double coverage of shingles on low-slopes....Then somewhere along the line it became to mean double felt....I may be crazy, but that's my recollection.

Wood shingles were installed in the same manner - Back when lower-slope porches were more common - The courses would shorten-up as the pitch decreased. Tore off plenty back in the day.

I don't know about the morphing, but around here, I seem to be the only roofer who laps the underlayment for double coverage on low slopes. I always did it because the directions said to do it. It makes sense, although it does not help a bit if the fastener goes through the cracks in a plank deck or otherwise does not hold into solid decking. That is the major problem with low slope comp roofing here in N. Texas.